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PubMed1995

The mechanism of lenticular accommodation in chicks.

In the chick eye, accommodation for near objects is brought about by changes in the focal length of the lens and by changes in the corneal radius of curvature. Several different mechanisms of lenticular accommodation have been proposed for the avian eye. These include a role for the ciliary muscle, a role for the iris muscle, and a role for changes in intraocular pressure. We have studied accommodation in the chick eye using electrical stimulation of the Edinger-Westphal nucleus, electric-field stimulation of enucleated eyes, in vitro measurement of changes in back vertex distance of the lens, and histology. We present evidence showing that, in the chick eye, lenticular accommodation is induced primarily by a contraction of the muscle fibers at the peripheral edge of the iris. During accommodation, the peripheral muscle fibers of the iris contract to apply a force through the ciliary processes to the anterior equatorial surface of the lens. This increases the focal power of the lens. When accommodation is relaxed, the lens is returned to its unaccommodated state by the elasticity of the pectinate ligament and the ciliary body. Contractions of the posterior ciliary muscle and changes in intraocular pressure, forces that have previously been proposed to play major roles in lenticular accommodation, are shown to be of secondary importance only.

Accommodation, Ocular↗
PubMed1995

The effect of topical beta-adrenoceptor antagonists on accommodation in emmetropia and myopia.

Using topical instillation of non-selective and selective beta-adrenoceptor antagonists we examine the proposal that a deficit in inhibitory sympathetic innervation of ciliary smooth muscle may be a specific precursor to the onset and development of late-onset myopia (LOM). Post-task accommodative hysteresis, a possible consequence of such a deficit, was assessed by measuring the time-course of regression of accommodation when open-loop (darkness) conditions were immediately imposed following far and near tasks. For the proposal to be feasible only LOMs should exhibit post-task responses which fail to differentiate the type of beta-antagonist employed. As the overall profile of responses to beta-adrenoceptor antagonism was equivalent for each of the three different refractive groups (emmetropes N = 6; early-onset myopes N = 5; LOMs N = 5) we conclude that a propensity to LOM is not associated with a deficit in sympathetic inhibition.

Accommodation, Ocular↗
PubMed1995

Monochromatic aberrations and myopia.

The monochromatic aberrations present in the eyes of a group of 21 young myopic subjects and 16 young emmetropic subjects were measured along the visual axis at three levels of accommodation. The aberrations were measured using a modified aberroscope technique which makes use of a retinal camera to photograph the shadow image of the aberroscope grid on the retina, while accommodation levels of 0, 1.5 and 3.0 D were induced consensually. Fourth-order aberrations were significantly different between the emmetropic and myopic groups, with the myopes showing lower fourth-order terms. A high proportion of the aberroscope grids photographed in the myopic eyes were too highly aberrated to permit analysis.

Accommodation, Ocular↗
PubMed1995

A dynamic relationship between myopia and blur-driven accommodation in school-aged children.

Previously we reported that recently myopic children accommodated insufficiently to blur induced by negative lenses. The purpose of the present study was to relate changes in blur-driven accommodation to myopia development in children. Refractive errors and the accommodation response function (ARF) were measured in 23 myopic and 40 emmetropic children on two occasions separated by periods ranging from 6 to 12 months. Repeated measures of accommodation were made with a Canon R-1 autorefractor while negative lenses of increasing power were placed in front of the child's right eye viewing 20/100 letters at 4 m. Concomitant changes in refractive error and in accommodative function over periods of 6-12 months were found to be highly correlated in myopes (r = 0.77) but not in emmetropes (r = 0.09).

Accommodation, Ocular↗
PubMed1995

In vitro changes in back vertex distance of chick and pigeon lenses: species differences and the effects of aging.

We have used a scanning laser technique to measure in vitro changes in back vertex distance of chick and pigeon lenses. Enucleated eyes were dissected, leaving the lens naturally suspended by the ciliary body and intraocular muscles. Ray tracing techniques were used to measure the resting back vertex distance of the lenses by passing a laser beam through the lens and scanning it across the pupillary aperture. The pupil diameter was measured videographically. The measurements were repeated while the intraocular muscles were pharmacologically stimulated with increasing concentrations of either nicotine or carbachol. Drug stimulation caused changes in pupil diameter and changes in the back vertex distance of the lenses. These experiments were conducted on the eyes of young chicks, young pigeons, and on the eyes of three pigeons older than 10 yr. The lenses from the eyes of the old pigeons had the greatest resting back vertex distance, followed by those of the young pigeons and finally those of the young chicks. Lenses from the eyes of young chicks and young pigeons underwent similar drug-induced changes in back vertex distance, but the lenses from old pigeon eyes showed an almost complete absence of such changes. Further, we demonstrated that, just as in the chick eye, lenticular changes in pigeon eyes are due to a contraction of the iris muscle. This is evident because after the iris has been removed the lens undergoes no change in back vertex distance during stimulation. We conclude with a discussion of the lenticular accommodative ability of the pigeon eye with reference to the recently reported accommodative mechanism of the chick eye and a comparison of chick and pigeon iris morphology.

Accommodation, Ocular↗
PubMed1995

Accommodation responds to changing contrast of long, middle and short spectral-waveband components of the retinal image.

We simulated the effects of longitudinal (axial) chromatic aberration and defocus on contrast of the long-, middle- and short-wavelength components of the retinal image to determine whether the effects of chromatic aberration are sufficient to drive accommodation. Accommodation was monitored continuously while subjects (12) viewed a 3 c/deg white sine-wave grating (0.92 contrast) in a Badal stimulus system. The contrasts (amplitudes) of the red, green and blue components of the white grating changed independently to simulate a grating oscillating from 1 D behind the retina to 1 D in front of the retina at 0.2 Hz. Subjects responded strongly to the chromatic simulation but poorly to a luminance control. The results support the hypothesis that focus is specified by the contrast of spectral-wavebands of the retinal image, and that conventional color mechanisms, monitoring chromatic contrast at luminance borders (1-8 c/deg), mediate the signals that specify dioptric vergence.

Accommodation, Ocular↗
PubMed1994

Lens accommodation evoked by microstimulation of the superior colliculus in the cat.

Accommodative responses to microstimulation of the superior colliculus (SC) of cats were investigated by measuring dioptric changes of the eye with a high-speed infrared optometer. Lens accommodation was elicited by low-current stimuli (< or = 20 microA) of the rostral portion of the SC, which corresponds to the representation of the central visual field. The low-threshold area for evoking lens accommodation was distributed from the superficial to intermediate layers of the SC. The latency of accommodative responses was 198.3 +/- 34.6 msec (mean and SD). The duration of accommodation was highly correlated with the duration of stimulation. These findings suggest that the SC plays an important role in the control of lens accommodation.

Accommodation, Ocular↗
PubMed1994

Spatiotemporal transfer function of human accommodation.

We have investigated the spatiotemporal transfer function of human "reflex" accommodation. An accommodative mechanism that is sensitive to an intermediate temporal rate of retinal image contrast change is proposed as the basis of the fine focus control hypothesis. To test the proposed mechanism accommodative responses were monitored by a dynamic infrared optometer while the subject focused on sinusoidal gratings (0.98-10.5 c/deg) which were moving sinusoidally at temporal frequencies in the range of 0.05-0.80 Hz over a 0.50 or 2.00 D peak-to-peak amplitude. The accommodative responses were best at 3 and 5 c/deg at both amplitudes of target motion. This result does not support the proposed mechanism or the fine focus control hypothesis for "reflex" accommodation. Fitting the data with first-order response functions showed little evidence of prediction. In addition, a second experiment found that the profile of the accommodative gain function is not altered by instruction at spatial frequencies above 5 c/deg in this type of dynamic accommodation experiment. The use of sinusoidally moving accommodative blur targets, particularly with careful instruction, seems to discourage voluntary accommodation in investigations of "reflex" control mechanisms of accommodation.

Accommodation, Ocular↗
PubMed1994

A simple mechanism for emmetropization without cues from accommodation or colour.

We propose and test the simple hypothesis that a chicken eye can emmetropize without cues derived from accommodation or colour just by maximizing retinal image contrast. Using different translucent occluders with known modulation transfer functions we found that deprivation myopia is correlated with the amount of image degradation. Equipped with a long-term integrator, a mechanism minimizing image degradation by changing the axial eye growth rate would therefore be sufficient to place the plane of focus of the eye at the average viewing distance.

Accommodation, Ocular↗
PubMed1994

In vivo determination of the biomechanical properties of the component elements of the accommodation mechanism.

A method of obtaining in vivo information about the component elements of the accommodation mechanism is described. Changes in axial lens thickness during accommodation are measured continuously by ultrasonographic biometry. The measurements are fitted with a dynamic biomechanical model of the mechanism of accommodation. This gives a time constant that depends both on the direction of accommodation and on the properties of the lens, zonules and choroid. Two ratios can be calculated that give information about the visco-elastic properties of the lens, zonules and choroid. This information is especially useful to evaluate the changes that lead to presbyopia.

Accommodation, Ocular↗
PubMed1994

The mechanism of corneal accommodation in chicks.

Corneal accommodation can account for up to 9 D of accommodation in a freely behaving chick. We have explored the possibility that changes in corneal curvature are due to changes in intraocular pressure (IOP) during accommodation. In an in vitro preparation we demonstrate that increasing the pressure will tend to flatten the cornea. We have used electrical stimulation of the Edinger-Westphal (EW) nucleus to further test the pressure hypothesis in vivo by recording IOP changes in the eye during EW-stimulated accommodation and by artificially modulating the IOP to assess the effects on corneal curvature. During EW stimulation there is an increase in IOP on the order of 1-3 mmHg which tends to flatten the curvature of the cornea, thus eliminating changes in IOP as a possible mechanism of corneal accommodation. Slit-lamp observations of accommodative changes at the corneo-scleral margin and electrical stimulation of dissected eyes in vitro indicate that corneal accommodation is mediated by a contraction of the ciliary muscles, which exerts a pull on the inner lamella of the cornea, flattening the peripheral cornea and increasing the curvature of the central cornea. Histological examination of the ciliary region of the eye confirms the appropriate positioning of the ciliary muscles. We conclude that corneal accommodation in the chick eye is accomplished by a ciliary muscle-mediated mechanism.

Accommodation, Ocular↗
PubMed1994

Aging of the human crystalline lens and anterior segment.

Changes in the unaccommodated human crystalline lens were characterized as a function of subject age for 100 normal emmetropes over the age range 18-70 yr by Scheimpflug slit-lamp photography. With increasing age, the lens becomes thicker sagittally, but since the distance from the cornea to the posterior lens surface remains unchanged, this indicates that the center of lens mass moves anteriorly and the anterior chamber becomes shallower. Sagittal nuclear thickness is independent of age, but both anterior and posterior cortical thicknesses increase with age, shifting the location of the nucleus and the central sulcus in the anterior direction. The amount of light scattered by the lens at high angles, as represented by normalized and integrated lens densities from the digitized images, increases with increasing age in an exponential fashion. Similar relationships to age are observed for the major anterior zone of discontinuity (maximum density) and the central sulcus (minimum density). The relationships of these results to accommodation and presbyopia are discussed.

Accommodation, Ocular↗
PubMed1994

Dark vergence in relation to fixation disparity at different luminance and blur levels.

Fixation disparity was measured psychophysically when subjects viewed a central fusion stimulus at 50 cm distance. Reducing luminance and increasing target blur resulted in an exophoric shift of fixation disparity. Fixation disparity was significantly more exophoric in subjects with distant dark vergence, i.e. the resting position of vergence. This was true for the bright and sharp target, as well as for targets impaired by blurring or dimming. These results are interpreted as a bias of fixation disparity towards the individual resting position of vergence not only for degraded stimuli but also for strong central fusion stimuli.

Accommodation, Ocular↗
PubMed1995

Moving the retina: choroidal modulation of refractive state.

The chick eye is able to change its refractive state by as much as 7 D by pushing the retina forward or pulling it back; this is effected by changes in the thickness of the choroid, the vascular tissue behind the retina and pigment epithelium. Chick eyes first made myopic by wearing diffusers and then permitted unrestricted vision developed choroids several times thicker than normal within days, thereby speeding recovery from deprivation myopia. Choroidal expansion does not occur when visual cues are reduced by dim illumination during the period of unrestricted vision. Furthermore, in chick eyes presented with myopic or hyperopic defocus by means of spectacle lenses, the choroid expands or thins, respectively, in compensation for the specific defocus imposed. Consequently, when the lenses are removed, the eye finds its refractive error suddenly of opposite sign, and the choroidal thickness again compensates by changing in the opposite direction. If a local region of the eye is made myopic by a partial diffuser and then given unrestricted vision, the choroid expands only in the myopic region. Although the mechanism of choroidal expansion is unknown, it might involve either a increased routing of aqueous humor into the uveoscleral outflow or osmotically generated water movement into the choroid. The latter is compatible with the increased choroidal proteoglycan synthesis either when eyes wear positive lenses or after diffuser removal.

Accommodation, Ocular↗
PubMed1995

Natural problems for stereoscopic depth perception in virtual environments.

The use of virtual reality (VR) display systems has escalated over the last 5 yr and may have consequences for those working within vision research. This paper provides a brief review of the literature pertaining to the representation of depth in stereoscopic VR displays. Specific attention is paid to the response of the accommodation system with its cross-links to vergence eye movements, and to the spatial errors that arise when portraying three-dimensional space on a two-dimensional window. It is suggested that these factors prevent large depth intervals of three-dimensional visual space being rendered with integrity through dual two-dimensional arrays.

Accommodation, Ocular↗
PubMed1995

Absence of adaptive plasticity after voluntary vergence and accommodation.

Subjects maintained their eyes crossed (verged) for a period of 8 min in darkness with monitoring provided by an infrared video system. Changes in resting vergence (RV) and resting focus (RF) were examined. Results showed: (i) visual stimulation was not necessary for adaptation of either RV or RF, but (ii) these purely motor effects were significantly smaller and more dissipative than those attributable to visually driven adaptation, and (iii) voluntary vergence amplitude was negatively correlated with pupil size. Assuming that voluntary vergence is driven by accommodation, then the voluntary signal must enter the oculomotor control system prior to the cross links between channels, but beyond the site of the visually driven adaptive elements.

Accommodation, Ocular↗
PubMed1996

Accommodative vergence is driven by the phasic component of the accommodative controller.

Conflicting opinions exist as to whether the phasic (reflex) component alone or both the phasic and tonic (adaptive) components of the accommodation and vergence systems drive accommodative vergence and vergence accommodation crosslinks. In this study the dissociated phoria to a 2 D target was measured before and after accommodative adaptation to discriminate the two possibilities. Results showed a significant difference in the dark-focus of accommodation pre- and post-near-vision task, indicating that accommodative adaptation had occurred. No significant change occurred in dark-vergence or in the accommodative response to the 2 D target. However, a significant decrease was found in the dissociated phoria presumably because of decreased phasic accommodation and its stimulation of accommodative vergence after the adaptation. This result is consistent with a model in which the accommodative vergence crosslink is driven by phasic accommodation only.

Accommodation, Ocular↗
PubMed1995

An adaptable association between vertical and horizontal vergence.

Vertical phoria (vergence error under monocular viewing conditions) can be trained to vary with conjugate eye position. The adaptive response controls the vertical alignment of the two eyes in the absence of binocular disparity and is used to compensate for binocular changes of the oculomotor system induced by developmental and environmental factors. Vertical phoria was associated with horizontal disparity vergence by adapting vertical vergence to two vertically disparate targets separated along the depth axis. This association was primarily dependent on the horizontal vergence as opposed to monocular eye position or binocular conjugate eye position. Following this adapted association with horizontal disparity vergence, vertical phoria aftereffects were also evoked by accommodative vergence. Previous reports have demonstrated an adapted association between vertical phoria and conjugate eye position. The current report examines the difference in the vertical phoria resulting from adaptation to vertically disparate targets separated along either the vertical axis or depth axis. The amplitude of the vertical vergence aftereffect was approximately 4 times greater for targets separated along the depth axis than in the vertical meridian. The association between vertical phoria with conjugate eye position and horizontal vergence is proposed to result from a cross-coupling of vertical vergence with supranuclear regions that control conjugate and horizontal vergence eye movements. A selective interaction would enable the oculomotor system to correct disturbances in specific supranuclear regions as they interface with vertical vergence.

Accommodation, Ocular↗